How does deep water drilling work?
The first offshore drilling platform was built in 1897 in California at the end of a wharf, according to How Stuff Works. This was followed by the first mobile platform for drilling in wetlands, developed in 1928 by a Texas oilman. It was simply a barge with a drilling rig mounted on it, but the start of a much larger offshore drilling industry that continued to accelerate. A platform that was far enough off the shore it couldn’t be seen from land was built in 1947 by a consortium of oil companies. These platforms weren’t just built to see if it could be done - as it turns out, most of the world’s petroleum is deep down, 500 to 25,000 feet below the surface of the Earth. New, giant oil reserves are being found in locations like the Gulf of Mexico at ever greater depths. As these deep water reservoirs are discovered, they are sometimes shrugged off as too deep to tap, but new drilling technology has allowed engineers to drill deeper and find huge amounts of hydrocarbons. Thousands of feet down, millions of years of decaying plant and animal matter has turned into three different forms of petroleum: liquid (oil), gas (natural gas), and solid (oil shale, tar sands).
Semi-submersible like those used for deepwater drilling. Image from Transocean.
How do drillers find petroleum deposits?
There are a few ways petroleum deposits are found: sniffers, magnetic surveys, gravity surveys, and seismic surveys. Sniffers detect natural gas traces in seawater, but are uncommonly used as they only detect deposits that leak, not those that are sealed hundreds or thousands of feet underground. Magnetic surveys allow a ship to map magnetic anomalies in rock density that affect the Earth’s magnetic field. These discrepancies can also be found via aircraft, which allows huge areas to be surveyed quickly. Canada even offers gravity and magnetic survey data from the government for free. When performing these surveys, surveyors are looking for nonuniformities in the rock that mean there could be an anomaly like oil and gas deposits. Geomagnetic field information has be updated as it is frequently changing, while the gravity field is vertical and goes one direction, making gravity surveys much more simple. Both of these survey types are great for large areas and almost always require other forms of surveying to compliment them, such as seismic surveys. For this reason, sometimes magnetic and gravity surveys are skipped in favor of seismic surveys alone, but technology like a new microelectromechanical (MEMS) gravity survey device the size of a postage stamp could revolutionize this area. A gravity survey might require $100,000 for equipment, plus ship or airplane time to use gravimeters, but this new technology could reduce the cost of the equipment to a few thousand dollars, while making it light and small enough to create a whole fleet of small autonomous survey ships or drones to cover large areas.
MEMS gravity survey device, the size of a postage stamp. Image from IEEE Spectrum.
Seismic surveys are by far the most common type of survey for petroleum. Acoustic shock waves are sent down into the ocean with air guns and read on hydrophones as they travel back to a moving vessel. The sound waves travel at different speeds through different types of rock and other subterranean materials, allowing scientists to then analyze that data to find deposits. Once deposits are detected, offshore areas can be leased from the governments that own them, and exploratory drilling can begin.
The initial wells to explore drilling for oil are called “wildcat” wells and can be drilled by different types of vessels containing drill rigs. In shallow waters 20 to 400 feet deep, a jack-up would most likely be used, which is a vessel with legs that can be extended below the vessel onto the sea floor, raising the hull above the waves to create a stable platform. In deeper waters, a semi-submersible platform can be either moored to the bottom of the ocean with cables or a tower, or stay in place with dynamic positioning. Although they may have thrusters, they don’t usually travel under their own power and need to be towed to new locations. Drillships operate in deep water and relocate quickly under their own power, good for drilling multiple exploratory wells in less time. Drillships usually have dynamic positioning systems as well, and similar to DP systems on a semi-submersible, the system is computer-controlled and uses azimuth thrusters to keep the vessel on station without the use of anchors or mooring. OneStep Power tests the electrical systems on dynamically positioned vessels to ensure fault ride-through, something you want to know if you’re relying on those systems for station-keeping while drilling a well!
“The Deepwater Proteus, a newbuild drillship that operates on hybrid-power systems which increase fuel efficiency. Source: Transocean”. Image from Journal of Petroleum Technology.
When drilling starts, core samples are removed and taken back up to the vessel, where geologists look for signs of petroleum, called a “show”, according to How Stuff Works. When a deposit is found, a production well is drilled and an offshore platform is moved into place to produce oil. Wells usually last about 10 to 20 years, so these platforms have to survive storms and all sorts of inclement weather without moving. The North Sea is a popular area for drilling and has very harsh weather with waves over 20 feet and wind speeds over 100 mph, according to Energy Voice. They’re usually attached to the sea bed with cables or metal legs. Multiple wells can be attached to one platform, and directional drilling means wells do not have to be drilled vertically but can be drilled at an angle miles away from the platform.
Huge waves and high winds at a platform in the North Sea. Image from Energy Voice.
How are deep water wells drilled?
Deepwater wells are usually drilled above the deposit, if possible. The first step is lowering a drill bit down to start the hole, also called “spudding in” a well. The drill bit spins inside a casing that ensures the surrounding soft sediment doesn’t cave in. The pipe is jetted into place with water or drilling fluid so the well head is above the sea floor, acting as a base for drilling. The drill bit continues down into the sediment and rock and drilling mud is pumped down through the bit to cool and lubricate it while also raising cuttings and particles from the drilling process to the surface through the hole and casing. The fluid also equalizes pressure between the inside and outside of the pipe and helps to keep fluids from flowing into the well bore, according to Shell. Mud is made up of water, clay, and fine rock, and is environmentally friendly.
Drill bit for drilling into hard rock. Image from AGU Advancing Earth and Space Science blog.
At a specified depth, a new casing pipe is run down into the well and concrete is pumped down into the pipe, flowing up around the outside of it to secure it in place. Mud is pushed down behind the cement and they are separated with a plug. In extremely deep wells, a second, smaller casing string is used and the same cementing process done to ensure strength. Each time a new casing is cemented in place, the bore is smaller than before. Before oil and gas is reached or pressures get too high, a blow out preventer, also called a BOP stack is secured to the sea floor with a riser on top. It contains valves to protect the rig and environment if the pressure is too great for the mud. The BOP stack is then tested and drilling continues with drilling done through the stack, with a final casing string installed once oil and gas are reached. Check out the video from Shell below for some great visuals on this process!
Video from Shell.
Once all the petroleum is flowing, it has to get where it needs to go. Pipelines deliver oil and gas to production platforms and to shore. Infield pipelines transport fluids inside the field to the platform and move processed water to injection wells. Export pipelines transport processed oil and gas to the shore from the platform in a multi-phase pipeline with both oil and gas, or a single-phase pipeline with just one. Transmission pipelines can carry oil and gas from one coast to another for trading.
Diagram of a deepwater drilling operation. Image from Wired.
Deep water drilling has gone far from where it started on a dock in California to platforms like small cities fixed in the ocean. Crews of over a hundred people work to pump hydrocarbons from 40,000 feet below the sea bed where oil is over 400 degrees Fahrenheit, according to Wired. Technology will continue to advance and previously unreachable deposits of oil and gas will be tapped by new rigs. We look forward to keep these rigs safe!
Be sure to check out the Chikyū world record drillship!